Rename DLLEXPORT AND STDCALL

This CL renames DLLEXPORT to FPDF_EXPORT and STDCALL to FPDF_CALLCONV to
be more PDFium specific.

This is split off of https://pdfium-review.googlesource.com/c/8970 by
Felix Kauselmann.

Bug: pdfium:825
Change-Id: I0aea9d43f1714b1e10e935c4a7eea685a5ad8998
Reviewed-on: https://pdfium-review.googlesource.com/10610
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/public/fpdf_flatten.h b/public/fpdf_flatten.h
index c1e354e..614540e 100644
--- a/public/fpdf_flatten.h
+++ b/public/fpdf_flatten.h
@@ -35,7 +35,7 @@
 //
 // Currently, all failures return |FLATTEN_FAIL| with no indication of the
 // cause.
-DLLEXPORT int STDCALL FPDFPage_Flatten(FPDF_PAGE page, int nFlag);
+FPDF_EXPORT int FPDF_CALLCONV FPDFPage_Flatten(FPDF_PAGE page, int nFlag);
 
 #ifdef __cplusplus
 }  // extern "C"